1. Field of the Invention
This invention relates to an exhaust gas recirculation system for an automatic transmission vehicle, and more particularly to an exhaust gas recirculation system for a vehicle provided with an automatic transmission having a plurality of shift patterns.
2. Description of the Prior Art
There has been in wide use an automatic transmission in which gear-shifting is automatically effected in accordance with a shift pattern predetermined according to the vehicle speed and the throttle opening. Recently, there has been put into practice an automatic transmission in which gear-shifting is automatically effected in accordance with a shift pattern chosen from a plurality of shift patterns by the driver. For example, when a shift pattern in which upshift occurs at relatively low vehicle speeds is selected, fuel economy can be improved since the engine runs mainly in relatively low rpm ranges. On the other hand, when a shift pattern in which upshift occurs at relatively high vehicle speeds is selected, the engine runs mainly in relatively high rpm ranges and, accordingly, a powerful driving force can be obtained.
However, when the vehicle having a plurality of shift patterns is provided with an exhaust gas recirculation system, there arises a problem that the operating condition of the engine is adversely affected by the exhaust gas returned to the combustion chamber when the shift pattern in which upshift occurs at relatively low vehicle speeds is selected. That is, in the exhaust gas recirculation system, a part of the exhaust gas, which is inert, is returned to the combustion chamber to lower the combustion temperature when the combustion temperature is high, thereby reducing formation of NOx. However, when the amount of exhaust gas returned to the combustion chamber is too large, the operating performance of the engine is adversely affected. Therefore, the amount of exhaust gas to be returned to the combustion chamber must be controlled according to the operating condition of the engine. However, conventionally the amount of exhaust gas to be returned to the combustion chamber has not been controlled in accordance with the rpm range to be mainly used, and accordingly, when the shift pattern in which a relatively low rpm range is mainly used is selected, the operating condition of the engine is deteriorated since the operating condition of the engine is more apt to be affected when the engine runs at low rpm.